Many electrical entities of the aforesaid kind are known. Such an electrical entity can be a modular control system, for example, including a plurality of assemblies as devices which are intended where possible to be exchangeable during operation. For assemblies which are not installed in an environment where explosive gas mixtures or dust/air mixtures are present, this usually involves relatively straightforward implementation requirements in terms of the construction design and logical bus system. However, issues arise if the control system is located in an Ex environment. Here, the separation of the assembly or device from the system is only permissible if e.g. exclusively intrinsically safe current circuits are connected via the plug connectors which must be separated. In the case of a relay assembly, this is only possible for the logical part, i.e. the bus connection and the activation of the relay.
Voltages of 60 V to 250 V and currents of several amperes are usually permissible for the load current circuit. If such a load current circuit is separated at the plug connector during operation, the resulting sparks can cause an ignition of the surrounding explosive mixture.
Even if the current circuit has been safely disconnected it cannot be assumed that the relay contacts are really open. Sticking relay contacts could be detected via a safety relay, but it would still not be permissible to exchange the relay assembly under load in the case of a sticking contact pair. In this case it would first be necessary to disconnect the load voltage via an Ex-approved isolating switch installed in the same zone or, as the case may be, via a normal switch which is installed in a safe environment.
An additional expense would therefore be required in each case, said expense being partially associated with personnel having special authorizations.